Potash, caustic, effect of the heating of, by an electric discharge, i. 84 its effect on electricity in a vacuum tube, i. 86 cyanate of, combination forming, i. 106 nitrate of, its opacity to the invisible rays, i. 65 oxalate of, formation of, i. 117 violet coloured light obtained by the combustion of, i. 133 in the land plants, i. 414 Potassium, i. 3 affinity for oxygen, i. 96 atomic weight of, compared with that of hydrogen, i. 100 one of an isomeric triad with cæsium and rubidium, i. 105 iodide of, whence obtained, i. 19 Potato, fungi causing the murrain in the, i. 284, 285 mode in which the injury is done, i. 284 poisonous nature of the leaves and berries of the, i. 426, 427 Poulpe, structure of the, ii. 245 Praya diphys, structure and mode of reproduction of, ii. 103-106 Propagation of diatoms by bisection, by conjugation, and by gonidia, i. 200 Protein produced by plants, i. 419 Protococcus pluvialis, structure and development of, i. 184 cycles of reproduction, i. 186 Protococcus viridis, abundance of, found in dust from Egypt, ii. 65 Protophytes, structure and development of, i. 182 Protoplasm of plants, structure and function of, i. 412, 413 Protozoa, structure of the, ii. 13 Pterideæ, characters of the group, i. 357 Pteris, characters of the genus, i. 357 Pteris aquilina, stems of, i. 339 or bracken, structure, mode of reproduction, and habitat of, i. 357, 358 Pteris esculenta of New Zealand, i. 358 Pteris serrulata, development of spores of, i. 337 antheridium and spermatozoids of, i. 338 Pteris serrulata: archegonium of, i. 338 Pucciniæ, structure and habitat of the sub-order, i. 276, 277 Puccinia Amorphæ, spores of, i. 276 Puccinia Fabæ, structure, habitat, and method of reproduction of, i. 280 alternation of generations of, i. 280 Puccinia Graminis, spore-cases of, i. 280 Puccinia lateripes, spores of, i. 276 Puff-balls of the meadows, i. 267 Purple dyes, i. 123, 124 Pycnogonoïdea, or spider crabs, structure and habitat of, ii. 211 Pyrosomidæ, structure and mode of reproduction of the, ii. 225 Pyxinei, characters of the, i. 307 Q Quartz, rock-crystal the purest form of, i. 17 certain thickness of, not transparent to invisible rays of light, i. 65 axis of symmetry of, i. 72 aqueous solution of, i. 110 characteristics of the, i. 111 Queen conch, shell of, ii. 234 Quinine, structure of, and whence obtained, i. 427 Quinqueloculina Bronniana, form of, ii. 228 R Radiation of light and heat, i. 34, 35 effects of, i. 35, 36 generally independent of colour, i. 36 Professor Tyndall’s experiments, i. 38 experiments showing radiation to be equal to absorption, i. 46 dynamic radiation, i. 49 absorption a phenomenon irrespective of aggregation, i. 53 Radicles, compound, i. 106